i have a 1990 4.9 with efi. about a week ago it started giving me problems starting when cold. i ran the codes and got 11 which means everything is good. my fuel pressure is 50 psi with key on and holds and 50+ psi when running. it will start but then stumble and die then i restart it and again it will die. once it warms up it idles and runs great. warm it will idle perfect. im stumped. cap and rotor are only a few years old and i checked them and they look good. i checked the coolant temp switch and it checked out. plug wires are fairly new. i tried cleaning the IAC with throttle body cleaner but made no difference. any ideas out there.

 

Sounds a lot like ignition plugs to me.

 

o2 sensors or the IAC is bad.... i'd lean more toward the IAC

 

Check fuel pressure in the morning, perhaps the key on and pump runs for 2-3 seconds isn't functioning or slow on the git go.

 

it's either the 2 wire temp sensor or the IAC....

the 2 wire temp sensor controls the cold start idle...and then the IAC does it...does it idle fine when warm...?

if it does then it is the 2 wire sensor...if it starts rough cold and wont idle right when warm it may be the IAC....

the 2 wire temp sensor is the cheaper part...

 

well i just replaced both the IAC and the coolant temp sensor. also checked the map and replaced it. ill see when it get cold tonight how it starts.

 

I know it has been over 3 years since this thread was active, I am just curious what the final result was. I have a issue just like this going on.

 

for some reason in the summer, when its above 100 degrees, it was doing it. i finally vented the gas caps and it stated perfect. cant figure it out so in the summer i just vent the caps and its perfect, it only does it in the summer, winter i can un-vent the caps.

 

Check the vent lines.

 

i did. thats what i couldnt figure out. they are vented. all i know is during the hot summer temps i end up venting the caps. winter theres no need.